Fibrocytes
Inflammation is the coordinated response to tissue injury or infection. The initiating events are mediated by local release of chemotactic factors, platelet activation, and initiations of the coagulation and complement pathways. These events stimulate the local endothelium, promoting the extravasation of neutrophils and monocytes. The second phase of inflammation is characterized by the influx into the tissue of cells of the adaptive immune system, including lymphocytes. The subsequent resolution phase, when apoptosis of the excess leukocytes and engulfment by tissue macrophages takes place, is also characterized by repair of tissue damage by stromal cells, such as fibroblasts.
In chronic inflammation, the resolution of inflammatory lesions is disordered, with the maintenance of inflammatory cells, fibroblast hyperplasia, and eventual tissue destruction. The mechanisms that lead to these events are complex, but include enhanced recruitment, survival and retention of cells and impaired emigration.
The source of fibroblasts responsible for repair of wound lesions or in other fibrotic responses is controversial. The conventional hypothesis suggests that local quiescent fibroblasts migrate into the affected area, produce extracellular matrix proteins, and promote wound contraction or fibrosis. An alternative hypothesis is that circulating fibroblast precursors (called fibrocytes) present within the blood migrate to the sites of injury or fibrosis, where they differentiate and mediate tissue repair and other fibrotic responses.
Fibrocytes are known to differentiate from a CD14+ peripheral blood monocyte precursor population. Fibrocytes express markers of both hematopoietic cells (CD45, MHC class II, CD34) and stromal cells (collagen types I and III and fibronectin). Mature fibrocytes rapidly enter sites of tissue injury where they secrete inflammatory cytokines. Fibrocytes are also capable of secreting extracellular matrix proteins, other cytokines and pro-angiogenic molecules, which may result in fibrosis.
Control of fibrocyte differentiation is likely to be important in the control of many diseases and processes. Fibrocytes are associated with a variety of processes and diseases including scleroderma, keloid scarring, rheumatoid arthritis, lupus, nephrogenic fibrosing dermopathy, and idiopathic pulmonary fibrosis. They play a role in the formation of fibrotic lesions after Schistosoma japonicum infection in mice and are also implicated in fibrosis associated with autoimmune diseases. Fibrocytes have also been implicated in pathogenic fibrosis, fibrosis associated with radiation damage, Lyme disease and pulmonary fibrosis. CD34+ fibrocytes have also been associated with stromal remodeling in pancreatitis and stromal fibrosis, whereas lack of such fibrocytes is associated with pancreatic tumors and adenocarcinomas. Fibrosis additionally occurs in asthma patients and possibly other pulmonary diseases such as chronic obstructive pulmonary disease when fibrocytes undergo further differentiation into myofibroblasts.
Fibrocytes may also play a role in a variety of conditions, likely even some in which fibrocyte formation is not currently known. Some additional conditions may include congestive heart failure, other post-ischemic conditions, post-surgical scarring including abdominal adhesions, corneal refraction surgery, and wide angle glaucoma trabeculectomy. Fibrocytes are also implicated in liver fibrosis and cirrhosis. See Tatiana Kisseleva et al, Bone Marrow-Derived Fibrocytes Participate in Pathogenesis of Liver Fibrosis, 45 Journal of Hepatology 429-438 (September 2006); see also F. P. Russo et al, The Bone Marrow Functionality Contributes to Liver Fibrosis, 130(6) Gastroenterology 1807-21 (May 2006). Fibrocytes are important in the formation of tumors, particularly stromal tissue in tumors. Recent evidence also suggests that fibrocytes may further differentiate into adipocytes and thus play a role in obesity.
Serum Amyloid P
SAP, a member of the pentraxin family of proteins that include C-reactive protein (CRP), is secreted by the liver and circulates in the blood as stable pentamers. The exact role of SAP is still unclear, although it appears to play a role in both the initiation and resolution phases of the immune response. SAP binds to sugar residues on the surface of bacteria leading to their opsonisation and engulfment. SAP also binds to free DNA and chromatin generated by apoptotic cells at the resolution of an immune response, thus preventing a secondary inflammatory response. Molecules bound by SAP are removed from extracellular areas due to the ability of SAP to bind to all three classical Fcγ receptors (FcγR), with a preference for FcγRI (CD64) and FcγRII (CD32). After receptor binding, SAP and any attached molecule are likely engulfed by the cell.
FcγR are necessary for the binding of IgG to a wide variety of hematopoietic cells. Peripheral blood monocytes express both CD64 and CD32 (a subpopulation of monocytes express CD16), whereas tissue macrophages express all three classical FcγR. Clustering of FcγR on monocytes by IgG, either bound to pathogens or as part of an immune complex, initiates a wide variety of biochemical events. The initial events following receptor aggregation include the activation of a series of src kinase proteins. In monocytes, these include lyn, hck and fgr, which phosphorylate tyrosine residues on the ITAM motif of the FcR-γ chain associated with FcγRI and FcγRIII, the ITAM motif within the cytoplasmic domain of FcγRIIa or the ITAM motif with the cytoplasmic domain of FcγRIIb. Phosphorylated ITAMs lead to the binding of a second set of src kinases, including syk. Syk has been shown to be vital for phagocytosis of IgG-coated particles. However, the wide distribution of syk in non-hematopoietic cells and the evidence that syk is involved in both integrin and G-protein coupled receptor signaling, indicates that this molecule has many functions.
Both SAP and CRP augment phagocytosis and bind to Fcγ receptors on a variety of cells. CRP binds with a high affinity to FcγRII (CD32), a lower affinity to FcγRI (CD64), but does not bind FcγRIII (CD16). SAP binds to all three classical Fcγ receptors, with a preference for FcγRI and FcγRII, particularly FCγRI. Although there are conflicting observations on the binding of CRP to FcγR, both SAP and CRP have been shown to bind to Fc receptors and initiate intracellular signaling events consistent with FcγR ligation.
In human blood serum, males normally have approximately 32 μg/ml+/−7 μg/ml of SAP, with a range of 12-50 μg/ml being normal. Human females generally have approximately 24 μg/ml+/−8 μg/ml of SAP in blood serum, with a range of 8-55 μg/ml being normal. In human cerebral spinal fluid there is normally approximately 12.8 ng/ml SAP in human males and approximately 8.5 ng/ml in females. Combining male and female data, the normal SAP level in human serum is 26 μg/ml+/−8 μg/ml with a range of 12-55 μg/ml being normal. (The above serum levels are expressed as mean+/−standard deviation.)
IL-12
IL-12 has been previously implicated in fibrosis and fibrosing diseases, but most studies have focused on the role of IL-12 in promoting the Th1 immune response or by triggering the production of interferon-γ. The direct effects of IL-12 on fibrocyte formation do not appear to have been previously recognized.
Laminin-1
Laminins are extracellular matrix proteins involved in movement of monocytes from the circulation into tissues. In order for leukocytes to enter tissues, they must cross through endothelial cells and the surrounding basement membrane of blood vessel wall. This process involves the tethering, rolling and stopping of the leukocytes on the endothelial cells. Following adhesion to the endothelial cells, leukocytes then cross between the endothelial cells, through the blood vessel wall and into the tissues. The process of extravasation of cells through blood vessel walls alters their phenotype and function.
These events are controlled by a series of cell surface adhesion receptors, including integrins. Integrins bind to a wide variety of ligands, including extracellular matrix proteins (ECM), such as fibronectin, vitronectin, collagen and laminin. Matrix proteins are present within the basement of the blood vessel wall, including laminins. Laminin are a large family of glycoproteins, with a heterotrimeric structure of α, β and γ chains. The use of different α, β and γ chains leads to the expression of at least 12 different laminin isoforms. Different laminins are expressed at different stages of development and at different sites within the body.
Scleroderma
Scleroderma is a non-inherited, noninfectious disease that has a range of symptoms. It involves the formation of scar tissue containing fibroblasts in the skin and internal organs. The origin of the fibroblasts is unknown. In mild or early cases of scleroderma, there is a hardening of the skin, fatigue, aches and sensitivity to cold. In more severe and later stages, there is high blood pressure, skin ulcers, difficulty moving joints, and death from lung scarring or kidney failure. Approximately 300,000 people in the U.S. have scleroderma. The disease has similarities to lupus and rheumatoid arthritis. There is no cure or significant treatment for scleroderma and even diagnosis is difficult because there is no clinical test.
Nephrogenic Fibrosing Dermopathy
Nephrogenic fibrosing dermopathy (NFD) is a newly recognized scleroderma-like fibrosing skin condition. It develops in patients with renal insufficiency. Yellow scleral plaques and circulating antiphospholipid antibodies have been proposed as markers of NFD. Dual immunohistochemical staining for CD34 and pro-collagen in the spindle cells of NFD suggest that the dermal cells of NFD may represent circulating fibrocytes recruited to the dermis. Therefore, inhibition of fibrocyte formation may alleviate symptoms of this disease.
Asthma
Asthma affects more than 100 million people worldwide, and its prevalence is increasing. Asthma appears to be caused by chronic airway inflammation. One of the most destructive aspects of asthma is remodeling of the airways in response to chronic inflammation. This remodeling involves thickening of the lamina reticularis (the subepithelial reticular basement membrane surrounding airways) due to fibrosis. The airway passages then become constricted due to the thickened airway walls.
The thickened lamina reticularis in asthma patients contains abnormally high levels of extracellular matrix proteins such as collagen I, collagen III, collagen V, fibronectin and tenascin. The source of these proteins appears to be a specialized type of fibroblast called myofibroblasts.
In asthma patients, CD34+/collagen I+ fibrocytes accumulate near the basement membrane of the bronchial mucosa within 4 hours of allergen exposure. 24 hours after allergen exposure, labeled monocytes/fibrocytes have been observed to express α-smooth muscle actin, a marker for myofibroblasts. These observations suggest that in asthma patients allergen exposure causes fibrocytes from the blood to enter the bronchial mucosa, differentiate into myofibroblasts, and then cause airway wall thickening and obstruct the airways. Further, there is a correlation between having a mutation in the regulatory regions of the genes encoding monocyte chemoattractant protein 1 or TGFβ-1 and the severity of asthma. This also suggests that recruitment of monocytes and appearance of myofibroblasts lead to complications of asthma.
Thickening of the lamina reticularis distinguishes asthma from chronic bronchitis or chronic obstructive pulmonary disease and is found even when asthma is controlled with conventional medications. An increased extent of airway wall thickening is associated with severe asthma. No medications or treatments have been found to reduce thickening of the lamina reticularis. However, it appears likely that reducing the number of myofibroblasts found in the airway walls may reduce thickening or help prevent further thickening.
Idiopathic Pulmonary Fibrosis
Idiopathic pulmonary fibrosis (IPF) is a unique type of chronic fibrosing lung disease of unknown etiology. The sequence of the pathogenic mechanisms is unknown, but the disease is characterized by epithelial injury and activation, the formation of distinctive subepithelial fibroblast/myofibroblast foci, and excessive extracellular matrix accumulation. These pathological processes usually lead to progressive and irreversible changes in the lung architecture, resulting in progressive respiratory insufficiency and an almost universally terminal outcome in a relatively short period of time. While research has largely focused on inflammatory mechanisms for initiating the fibrotic response, recent evidence strongly suggests that disruption of the alveolar epithelium is an underlying pathogenic event. Given the role played by fibrocytes in wound healing and their known role in airway wall thickening in asthma, it appears likely that overproduction of fibrocytes may be implicated in IPF.